Polyurethane and polyisocyanurate (PUR/PIR) foams are well known in the art and are conventionally made utilizing a blowing agent. It is known to use hydrocarbons as a blowing agent in the manufacture of such foam. However, the highly flammable nature of some blowing agents, such as pentanes makes it unattractive for commercial use due to the extra precautions needed to assure plant safety. It would be desirable to develop processes for the utilization of flammable blowing agents in the commercial production manufacture of PUR/PIR foams.